These radar systems consist of a transponder on land (interrogator) having a system of rotating antenna and a transceiver also on an aircraft (transponder) which basically utilizes an omni-directional antenna. The interrogator positioned on land typically emits three pulses (designated hereinafter P1, P2, P3) in succession, e.g. at a frequency of 1030 MHz, with, for example, peak power levels ranging between 0,5 and 4 KW.
The time interval between the pulses P1 and P3 radiated by means of a directional antenna, is related to the particular "mode" of interrogation. Pulse P2 which presents a fixed delay with respect to pulse P1, is radiated from a so-called "control" beam of the pseudo-multi-directional type, which has the function of avoiding interrogations from the lateral lobes of the directional beam (SLS--Side Lobe Suppression).
From this it follows that, at the input of the transponder the ratio between the amplitudes of pulses P1 and P2 turns out to depend on the azimuth of the aircraft referred to the axis of the directional antenna beam.
Basically the following two cases exist:
(a) for the azimuths included in a section of the directional beam, called the "effective beam" the ratio P1/P2 is nominally greater than 9 dB;
(b) for pulses arriving from the lateral lobes of the directional beam, the ratio P1/P3 is nominally less than 0 dB.
In the first case the transponder has to reply, in the second case there normally should be no response. What has been described is subjected to international regulations (ICAO, STANAG).
In practice various malfunctions are found to occur within the system and in its interaction with the environment.
In particular:
(a) Reflecting surfaces such as airport structures, ground aircraft, buildings, ships etc. are causes of partial deviation of the directional beam whereby interrogations of aircrafts can occur outside the path of the beam.
(b) The surroundings, in particular the radome structure often used to protect the antenna systems, tend to noticeably deform in an unpredictable fashion the two antenna beams.
The ratio P1/P2 therefore appears altered and for each of the angular sectors outside the directional beam, interrogations and thus undesired responses may take place.
(c) A weakness of the SSR/IFF radars consists in the lack of angular discrimination which creates problems, especially in the "terminal" regions where an elevated air-traffic density can be encountered.
Present-day interrogators are not entirely able to overcome the problems described above.
In an "on-site" optimization procedure, efforts are generally directed to reducing the transmission power. This is achieved by means of fixed power attenuators, placed in cascade at the transmitter. In other words, the general power of the transmitter is reduced by a fixed amount and in a discriminative way with respect to the specific directions from which the disturbance is expected.
As regards the reflections, optional units exist, which operate on the transmission pulses in such a way as to make the transponder insensitive to transmission by the lateral lobes (IISLS: Improved Interrogator Side Lobe Suppression).
The aforedescribed techniques are subject to the following limitations:
(a) little efficiency;
(b) the partial blindness imposed upon the transponder tends to diminish its ability to respond to another interrogating sensor;
(c) a significant amount of hardware which either causes an appreciable loss (at least 3 dB) or requires the addition of a second transmitter.